United has removed their sizers from gate areas, though you'll still find them near their check-in kiosks and counters. This is a good article to understand their approach:
https://www.sfgate.com/travel/article/United-luggage-sizers-dimensions-bags-15045784.php
"That doesn’t mean United is turning a blind eye to overstuffed luggage" but they are using common sense "eyeballing" rather than tape measurers and scales.
I've even noted that the United sizers at their check-in counters are slightly larger than their stated dimensions, with the "box" being just a bit bigger than where the arrows of the dimensions end. That's probably to allow for a bag that might exceed any single dimension, but still falls within the "overall" dimensions.
I have regularly flown United for decades and can't ever remember having my carry-on bags weighed or measured. And my structured backpack is right at the United personal item dimensions.
Lufthansa usually operates my European connections. They can be pickier, but still they have always just glanced at my carry-on backpack, never weighed or measured it. Perhaps if it were overlarge they might take more notice.